Friday, May 7, 2010

Renewable energy and hydrogen – what are the alternatives?

Bookmark Business Technology and People
The last couple of months were very busy and did not leave much place for creative writing. But I learned intensively about the state of the art in hydrogen based energy usage, in particular with automotive and off-grid applications.

One of the hottest debates in automotive is the merits of battery based vehicle compared to other forms (hybrid fuel-electric or hydrogen-electric). In the US we saw a major shift from hydrogen to battery, motivated by the expectations that battery powered vehicles are readily available and the least disruptive in terms of required infrastructure, while hydrogen is more complex technically and requires a new specialized infrastructure (assuming the use of compressed hydrogen as the storage option).

But let’s pause for a moment to look at the energy aspects of those alternatives – after all, that’s what we are trying to get in the first place. Our experience with electronic devices such as mobile phones and cameras made us pretty happy with modern batteries – they are small, lightweight and last hours or days before requiring a recharge. Why not put them in a car?
What we tend to overlook is that those electronic devices also made significant progress in lowering their energy requirements. But when considering a car, it’s another order of magnitude.
A cruising car at 90kmh consumes some 20kWh. Compact sized cars such as the Audi A3 have engines that deliver about 100kW, and for example the range of the BMW series 5 engines extends from 125kW to 270kW. In my camera, I have a tiny 5Wh battery that lasts forever and weighs only 28gr. Using that same technology to store 100kWh in a car would require 560Kg of batteries...
A common metric for state of the art car batteries (supporting over 1000 charge cycles) is a capacity of 120Wh per Kg of battery. Which explains why Nissan’s brand new battery powered car, the Leaf, is rated for a mere 160Km of autonomy.

What about hydrogen? In recent years, we saw quite a few real size tests of hydrogen vehicles. One of the best known is the Honda FCX in California, with 100kW engine, 450Km autonomy and 16 hydrogen filling stations in the Los Angeles area. Looks good, except that the hydrogen tank fills up the trunk of this mid-sized car (the image shows the rear part of the FCX chasis with the tank). While compressed hydrogen offers better energy density than batteries and is a mature and known technology, it has a physical limit – in its most dense form (liquid at near absolute zero), it contains only 2.2kWh (8MJ) per liter of volume; and in the Honda FCX the hydrogen is stored under pressure of 350 bars, containing only 800Wh (2.9MJ) per liter.

Hydrogen being the smallest gas molecule known poses its particular issues. It is a tremendous challenge for sealing (even welded/brazed joints still leak) not to mention any long term storage. A hydrogen leak in the atmosphere is much more dangerous than a natural gas leak due to the energy content (remember the Hindenberg?). The infrastructure requirements for compressed or liquid hydrogen based fuel are huge as well. Also, modern storage tanks for compressed hydrogen are made of carbon fibre, a very strong, light and expensive material...

Does that imply that electrical cars are doomed to low performance and limited autonomy? Not if we find better, more dense methods for storing hydrogen.

One such avenue uses, instead of physical pressure or extremely cold temperature, chemical forces to compress the hydrogen. It is long known that some compounds, called hydrides, bind multiple hydrogen atoms in a density much higher than liquid hydrogen. For example, Lithium Borohydride contains 12kWh/Liter (compared to gasoline with 10kWh/Liter and compressed hydrogen with less than 1kWh/Liter).
Practical research presently attempts to produce liquid fuel which could be used with the present fuel infrastructure, and has reached fuels with a density of 1.3kWh to 2kWh per liter. That's already better than compressed hydrogen and still leaves room for much progress and improvement.
But the use of this type of fuel is not as straight forward as compressed hydrogen, since it requires more manipulations. To obtain the hydrogen you need an on-board a fuel processor which, via a chemical reaction, liberates the hydrogen from the fuel. Additionally, the “spent” fuel has to be retrieved at the fuel station and resent to the “refinery” for regeneration (recharging with hydrogen).

What to make of it? Modern human society has grown accustomed to almost instant results in its daily affaires: fast food, micro-oven meals, high risk financial instruments with fast yields, video on demand, mobile internet and accessibility (blackberry...) are typical examples. Faced with mountig energy costs and pollution, the temptation is high to go for fast solutions. I believe that this is a key factor behind the decision of the current US administration to prioritize battery cars over hydrogen, and compressed hydrogen over chemical hydride solutions.
But if you take the time to look at the fundamentals, the ability to get a fast solution is of marginal value.

What we need is high energy density in a sustainable and renewable fuel, which can be stored and transported safely and conveniently and used whenever and wherever needed. The present state of the art points to stable liquid fuels made of chemical borohydrides as the most convenient alternative: simple and low cost charging, using the current fuel infrastructure for storage and transportation, and simple and low cost on-board processing.

I’m looking forward to your views.

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